1. Field of the Invention
The present invention relates to a data recording apparatus for cameras, and more particularly to a data recording apparatus for optically or magnetically recording various types of data on a photographic film while the film is advanced.
2. Description of the Related Art
It is known to utilize cameras capable of recording photographing data on a photographic film at the time of taking a photograph. The photographing data may relate to shutter speed, aperture value, electronic flash conditions, aspect ratio representing a print format, position of a main subject on a picture frame, or other variables. This photographing data is read from the photographic film during the photofinishing process for controlling the print exposure, setting the print format, and the like. A magnetic data recording method described, for example, in U.S. Pat. No. 4,864,332 and U.S. Pat. No. 5,021,820 and an optical recording method described, for example, in U.S. Pat. No. 5,023,656 are known. Digital recording is advantageous over analog recording in that errors in reading and writing the data can be minimized. When digital recording is used, data is represented by a binary code consisting of a plurality of bits, and the binary code may be written as a combination of magnetic areas and non-magnetic areas, or as a combination of photographically exposed areas and non-exposed areas.
If the film advance speed of the camera is always constant, a binary code representing photographing data can be written on the film loaded in the camera while the film is advanced, with bit lengths being constant, by driving the magnetic recording head or light recording head in accordance with the constant film advance speed. However, it is very difficult to maintain a constant film advance speed over the full length of a film roll, because the diameter of the film wound on the film take-up spool increases as the film is advanced frame by frame. Therefore, in order to record each bit of the binary code in a predetermined fixed length of film, it is necessary to drive the recording head in accordance with the precisely detected film advance speed.
Conventionally, the film advance amount is measured by an encoder constructed of an encoded disk and a photo-detector. The encoded disk has a plurality of slits formed therein at a fine pitch so as to extend in the radial direction. The encoded disk is coupled to the camera advance mechanism and rotates synchronously with the film advance. The photo-detector detects a passage of each slit. A film advance speed is detected from the number of slits which pass per unit time, and a film winding amount is detected from the total number of slits which have passed. Using these detected values, the recording head is controlled and driven.
As the amount of data written within a length of one frame increases, the bit record length (unit bit length) of each bit constituting a binary code must be shortened. Under such a condition, it is very difficult to write a binary code correctly and avoid the generation of read errors if the recording head is driven in the manner described above. To solve this problem, it is necessary to more accurately measure a film advance speed and winding amount through the use of an encoded disk having a finer pitch of slits. This requires a high precision in machining of the encoded disk, and a high resolution photo-detector. Such a construction is expensive, resulting in a high cost of manufacturing of the camera.